Speaker Mike Johnson on Wednesday faced a buzz saw of Republican opposition to his bid to extend a warrantless surveillance law that national security officials call crucial to their efforts to fight terrorism, after former President Donald J. Trump urged lawmakers to kill the legislation.

Republican leaders said they would plunge ahead with a midday vote to bring up the bill, which would extend a section of the Foreign Intelligence Surveillance Act known as Section 702 and make modest changes. But the fate of the measure was very much in doubt after Mr. Trump’s statement, which added a powerful voice of opposition to an already sizable contingent of right-wing lawmakers who have clamored for a more sweeping overhaul that would severely limit the government’s spying powers.

Aides said it was possible that Republicans would yank the bill if they failed to quell the brewing revolt.

No Democrats were expected to vote to move forward on the measure — among other things, Republicans have bundled it with an unrelated resolution condemning President Biden’s border policies — so just three Republican defections would be enough to scuttle the move. At least one hard-right member, Representative Matt Gaetz of Florida, has already pledged to try to tank it.

Stoking the opposition, Mr. Trump posted overnight on social media, “KILL FISA, IT WAS ILLEGALLY USED AGAINST ME, AND MANY OTHERS. THEY SPIED ON MY CAMPAIGN!!!”

The statement was largely incoherent as a matter of policy. Section 702 allows the government to target foreigners abroad for surveillance without warrants. The instance Mr. Trump was apparently referring to — when the F.B.I. obtained wiretap orders on a former campaign adviser to his 2016 campaign as part of an investigation into Russian interference — concerned a different section of FISA for targeting Americans and people on domestic soil in national security inquiries.

But as a matter of politics, Mr. Trump’s attack on the measure only underscored his lingering grievances about that investigation and his disdain for national security agencies he often disparages as an evil “deep state.” And it was resonating with his hard-right allies on Capitol Hill, who see blocking the extension of the law — which government officials say is crucial to their foreign intelligence and counterterrorism work — as a way to inflict pain on an intelligence community they regard as an enemy.

Section 702 is set to expire on April 19. But the program could continue operating until April 2025 if the FISA court grants a government request for orders authorizing it for another year before the underlying statute expires. As a result, if the House measure collapses, the next Congress could revisit it.

Mr. Trump’s intervention recalled a similar episode in early 2018, when he set off last-minute turmoil with a social media broadside against FISA just as House Republicans were scrambling to secure enough support to extend Section 702 before it expired, a move backed by his administration. Hours later, after lobbying by Speaker Paul D. Ryan, Mr. Trump walked back the statement in another social media post, and the bill passed.

Mr. Johnson, who previously opposed the pending legislation and backed a more sweeping overhaul, now says the bill contains “the most significant set of intelligence reforms since FISA was originally enacted in 1978.”

But Mr. Gaetz said he and others would not allow it to move ahead.

“I don’t think we should proceed on to this bill until we’ve got a better understanding of how to adhere to the Constitution,” Mr. Gaetz said, adding, “I don’t see the rule passing.”

Representative Anna Paulina Luna, Republican of Florida, echoed the sentiment in a social media post Wednesday morning, writing, “We are killing FISA,” and predicting that in its current form, the measure could not pass.

At issue is a debate that has roiled Congress for months. Under Section 702, the government is empowered to collect, without warrants, the messages of noncitizens abroad, even when those targeted are communicating with Americans.

As a result, the government sometimes collects Americans’ private messages without a warrant. While there are limits on how those messages can be searched for and used, the F.B.I. has repeatedly violated those limits in recent years — including improperly querying for information about Black Lives Matter protesters and people suspected of participating in the Jan. 6, 2021 Capitol riot.

The F.B.I. has since tightened its system to reduce the risk of queries that violate the standards, and the bill in question would codify those changes and add reporting requirements. It would also limit the number of officials with access to the raw repository of information collected.

But reformers — including both progressive Democrats and libertarian-minded Republicans — want to add a requirement that officials must get a warrant before querying the repository for information about an American. Under the rules to be voted on Wednesday, critics led by Representative Jim Jordan, Republican of Ohio and the chairman of the Judiciary Committee, would get a chance to try to add that requirement to the bill.

National security officials argue that doing so would cripple the program. Senior lawmakers on the House national security committees, including Representatives Michael R. Turner, Republican of Ohio and the chairman of the Intelligence Committee, and Jim Himes of Connecticut, its top Democrat, have also resisted such changes, and are backing the more modest adjustments in the bill.

But a handful of Republicans favor allowing Section 702 to expire altogether — Mr. Gaetz among them.

“I’m incredibly disappointed that the views that Speaker Johnson deeply held for seven years as he sat next to me on the House Judiciary Committee, he has done a 180 on,” Mr. Gaetz said. “Mike Johnson made the arguments against FISA and its abuses better than I did in the House Judiciary Committee. And this is something that I strongly disagree with.”

In a letter to fellow Republicans, Mr. Johnson laid out his reasons for pushing for the extension.

“FISA and Section 702 have been essential to intercepting communications of dangerous foreign actors overseas, understanding the threats against our country, countering our adversaries and saving countless American lives,” Mr. Johnson wrote.

“However, as a former constitutional law litigator and chair of the House Judiciary’s Subcommittee on the Constitution,” he added, “I can state unequivocally that the F.B.I. terribly abused the FISA authority in recent years, and in turn, violated the trust and confidence of the American people. Our responsibility now is simple: maintain the tool but strictly prohibit future abuses.”

Catie Edmondson contributed reporting.